Magnetic recording media, and optical recording media such as phase-change recording media, are portable recording media that allow large capacities of information to be recorded at high density, and as the trend toward multimedia has continued in recent years, there has been a surge in demand for media with which large-capacity computer files or moving pictures can be recorded.
An optical recording medium generally has a structure in which a multilayer thin film such as a recording film that includes a recording layer is formed on a transparent, disk-shaped substrate made of plastic or the like. This optical recording medium is irradiated with a laser, a focusing servo and guide grooves or pre-pits are used to record and erase information while applying a tracking servo, and the reflected laser light is used to reproduce information.
Conventional magnetic recording media have mainly been what is known as optical modulation recording systems, in which information that has been recorded by adding a static magnetic field is erased, after which new information is recorded by adding a static magnetic field in the opposite direction. More recently, however, attention has shifted to magnetic field modulation systems because they make possible recording in a single rotation (direct overwrite) and allow accurate recording even at high density levels. Also, phase-change optical recording media have been gaining popularity because they afford direct overwrite by employing optical modulation recording and allow reproduction with the same optical system as a CD or DVD.
The recording density limit of an optical recording medium is determined by the laser wavelength (λ) of the light source, and is a function of the diffraction limit (≈λ/(2NA); where NA is the numerical aperture of the objective lens). In recent years a system has been proposed in which a NA of 0.8 or higher is attained by using a set of two objective lenses, and these systems have undergone considerable development. The laser used for recording and reproduction is directed through the substrate at a recording film that includes a recording layer, but as the NA increases, aberration caused by tilt of the substrate and so forth increases when light passes through the substrate, so the substrate has to be made thinner.
Also, with a magnetic recording medium, higher density recording than with optical recording media has been achieved by the development of practical magnetic recording heads (such as GMR heads) and improvements to the medium, but because there are no guide grooves or pre-pits for the tracking servo as there are with an optical recording medium, a tracking signal has to be prerecorded with a servo writer or the like.
A method for forming servo pits by magnetic transfer, for example, has been proposed as a way to solve the above problem (see Patent Document 1, for example).
Patent Document 1: Japanese Unexamined Patent Publication H10-40544